1. Field of the Invention
The present invention relates to a poly-phase filter of a terrestrial digital multimedia broadcasting (T-DMB) receiver in a low intermediate frequency (IF) structure applied to a mobile communication terminal such as a cell phone, and more particularly, to a poly-phase filter capable of maintaining a desired value of electrical characteristics of the poly-phase filter regardless of changes in a manufacturing process or temperature.
2. Description of the Related Art
A conventional poly-phase filter applied to a receiver in a low intermediate frequency (IF) structure is described with reference to FIG. 1.
FIG. 1 is a circuit diagram of a conventional low IF receiver.
Referring to FIG. 1, the conventional low IF receiver 100 includes an antenna 110, an amplifier 120, a voltage-controlled oscillator 170, a divider 160, a mixing unit 130, an image rejection channel filter 140, and an IF amplifier 150.
The antenna 110 receives a radio frequency (RF) signal.
The amplifier 120 minimizes noise in the RF signal received by the antenna 110 and amplifies the RF signal. Here, in some cases, the RF signal is amplified by controlling a value of an amplification gain.
The voltage-controlled oscillator 170 generates an oscillating frequency signal having a predetermined frequency.
The divider 160 divides the oscillating frequency signal to generate a plurality of local oscillating frequency signals which are to be used for frequency synthesizing with the amplified RF signal.
The mixing unit 130 includes a first mixer 131 and a second mixer 132. The first mixer 131 mixes an output signal of the amplifier 120 with a first local oscillating frequency signal including two signals which are generated by the divider 160 and have a phase difference of 180°. The second mixer 132 mixes the output signal of the amplifier 120 with a second local oscillating frequency signal including two signals which are generated by the divider 160 and delayed by 90° than those in the first local oscillating frequency signal.
The image rejection channel filter 140 rejects image components in an IF signal output from the mixing unit 130. The IF signal from which the image components are rejected is output through the IF amplifier 150.
A function of the low IF receiver is to convert the RF signal into the IF signal. Here, the IF signal includes the image components in addition to desired real components, so that the low IF receiver has to have a function of rejecting the image components in order to perform accurate signal processing in following operations. In order to perform the function of rejecting the image components, the mixing unit 130 uses two mixers 131 and 132 and controls phases of two paths including an I-path through which an output signal of the first mixer 131 is transmitted and a Q-path through which an output signal of the second mixer 132 is transmitted. Here, the desired real components pass, and a phase of the image components in the I-path is opposite to that in the Q-path so that the phases are vanished. The function of rejecting the image components is performed by the image rejection channel filter 140.
However, a poly-phase processing filter circuit generally includes a resistor and a capacitor. When the poly-phase processing circuit is implemented as an integrated circuit (IC), the resistor and the capacitor can be easily implemented. However, a resistance of the resistor and a capacitance of the capacitor have process variations of 30% and 10%, respectively. When the resistance of the resistor and the capacitance of the capacitor are changed, frequency characteristic of the filter are also changed. In addition to the process variations, when temperature of the IC or temperature of a system including the IC is changed, the resistance of the resistor and the capacitance of the capacitor are also changed. As described above, in order to counter with causes of the changes in the frequency characteristics of the filter such as the changes in circumstances and the process variations, a scheme of designing a multistage poly-phase filter is proposed.
However, in order to deign the multistage poly-phase filter, a very wide range of frequencies has to be set so as to cover a practically desired frequency range, and this causes an increase in area.